1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program for communicating between a plurality of wireless stations such as in a wireless LAN (Local Area Network), and more specifically to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program for running a wireless network in which direct communications (random access) are made in an asynchronous mode between terminals. More in detail, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program in which an ad-hoc communication wireless network is established without particularly providing an apparatus acting as a control station, more specifically to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program in which each communication station is provided with its own transmission/reception period without causing useless latency.
2. Description of Related Art
By establishing a LAN with a plurality of interconnected computers, it is possible to share information such as files and data, peripheral equipments such as printers and the like, and also to exchange information such as transmission of electronic mails and data contents. Conventionally, it is typical to construct a cabled LAN, which requires laying cables, thereby making it difficult to construct networks with minimum expense. Additionally, after constructing such network, it encountered inconvenience because moving range of equipments was restricted by the cable length.
On the contrary, a wireless LAN is increasingly attracting attention as a system for releasing users from cabling in the cabled LAN. Such wireless LAN enables to eliminate almost all cablings in the working area such as in an office or the like, thereby making it possible to relatively easily move communication terminals such as personal computers (PCs) or the like. As a result of higher operating speed and lower price of a wireless LAN system in recent years, needs for such wireless LANs are significantly increasing. Particularly in recent years, an investigation for introduction of a personal area network (PAN) have been made for performing information communications between a plurality of personal electronic equipments and appliances with a small-scale wireless network.
For example, some different wireless communication systems have been standardized by utilizing such frequency bands as 2.4 GHz and 5 GHz which require no license from an administrative government office. One of the standardized wireless communication networks is the IEEE (The Institute of Electrical and Electronics Engineers) 802.11 or the IEEE 802.15.3. As for the IEEE 802.11 standard, there are various types of wireless communication systems such as the IEEE 802.11a, the IEEE 802.11b, or the like depending on wireless communication systems, frequency bands to be used, etc.
Also, in recent years, a so-called “ultra-wideband (UWB) communication” which carries out the wireless communications by putting information on very weak impulse series is attracting attention as a short range, ultra-high speed wireless communication system, and this system is expected to be made practicable.
As for the UWB transmission system, studies have been made on various physical signal types such as a DS-UWB system in which the spreading speed of a DS (Digital Service) information signal is increased to the extreme, an impulse-UWB system in which information signal is constructed by using very short period impulse signal series in the order of several hundreds picoseconds for transmission and reception of such signal series, etc. Any one of the systems utilizes ultra-wide frequency bandwidth in the range of, for example, 3 GHz to 10 GHz. For example, spread processing is performed within the frequency bandwidth for transmission and reception, thereby realizing high speed data transmission. Its occupied bandwidth is a GHz order in which the occupied bandwidth divided by the center frequency (for example, 1 GHz to 10 GHz) is equal to substantially 1. This is much wider bandwidth as compared to the bandwidth normally used in the wireless LAN such as a so-called W-CDMA, a cdma2000, an SS (Spread Spectrum), or an OFDM (Orthogonal Frequency Division Multiplexing) system.
In the studies for standardizing, for example, the IEEE 802.15.3, standardization is in progress on a method of communicating by forming a piconet between wireless communication apparatus which perform ultra-wideband wireless communication.
In order to construct a local area network using a wireless communication technology, it is typical to provide a single apparatus acting as a control station which is known as “an access point” or “a coordinator” in the area and the network is formed under supervising control of the control station.
In case of transmitting information from a certain communication apparatus in a wireless network having an access point, widely used is an access control method based on a reserved bandwidth in which a bandwidth necessary for transmitting the information is firstly reserved from the access point in order to use the transmission path so that no collision takes place with information transmission by another communication apparatus. In other words, the access point is provided to perform synchronized wireless communication in which all communication apparatus in the wireless network are synchronized with each other.
However, there arises a problem to significantly decrease efficiency of using the transmission path in case of performing asynchronous communication between a transmitter side and a receiver side in the wireless communication system having the access point because it is absolutely necessary to perform wireless communication through the access point.
On the contrary, as another method of constructing a wireless network, a so-called “ad-hoc communication” for directly performing asynchronous communication between terminals has been proposed. Particularly, in a small-scale wireless network comprising a relatively small number of clients that locate close to each other, it is considered that such ad-hoc communication is suitable because any terminal can directly perform asynchronous wireless communication without using a particular access point.
Now, a conventional wireless networking will be described hereunder in detail by taking the IEEE 802.11 as an example. The networking in the IEEE 802.11 is based on the concept of a BSS (Basic Service Set). The BSS comprises two kinds, one is a BSS as defined by an infrastructure mode including a master such as an AP (Access Point or a control station) and the other is an IBSS (Independent BSS) as defined by only an ad-hoc mode including a plurality of MT (Mobile Terminals or movable stations).
Infrastructure Mode:
An operation of the IEEE 802.11 in the infrastructure mode will be described by reference to FIG. 26. An AP to perform coordination is essential in a BSS in the infrastructure mode.
An AP integrates the area where a radio wave reaches in the periphery of its own station as a BSS and constitutes a so-called “cell” in a cellular system. MTs in the neighborhood of the AP are accommodated in the AP and enter the network as a member of the BSS. In other words, the AP transmits a control signal, which is known as a beacon, at an appropriate time interval. Any MT capable of receiving the beacon is considered to be located near the AP and establishes a connection with the AP.
In the example as shown in FIG. 26, a communication station STA0 operates as the AP and the other communication stations STA1 and STA2 operate as the MTs. Note that the STA0 as the AP transmits the beacon at a constant interval as show in the chart at the right side in FIG. 26. The transmission time of the next subsequent beacon is informed in the beacon as a parameter of target beacon transmit time (TBTT). And when it comes to the TBTT, the AP initiates the beacon transmission procedure.
Upon receiving the beacon, neighboring MTs decode the internal TBTT field for recognizing the next subsequent beacon transmission time, thereby making it possible to shut-off the power of the receiver and to enter the sleeping state until the TBTT for the next subsequent or some later TBTT depending on the case (or in case when reception is unnecessary).
Ad-Hoc Mode:
Now, the operation of the IEEE 802.11 in another or the ad-hoc mode will be described by reference to FIG. 27 and FIG. 28.
In the IBSS in the ad-hoc mode, MTs automatically define the IBSS after performing negotiation among a plurality of MTs. Upon defining the IBSS, a group of MTs determine the TBTT at a constant interval after such negotiation. Upon recognizing arrival of the TBTT by making reference to the clock within its own station, each MT transmits the beacon after a random time delay in case when it recognizes that no other MTs transmit the beacon.
The particular example as shown in FIG. 27 shows that two MTs constitute the IBSS. In this case, the beacon is transmitted by either one of the two MTs belonging to the IBSS at every time when the TBTT is reached. Also, there is a case when collision of beacon takes place.
Even in the IBSS, there is an instance when the power supply of the MTs is shut off as necessary, thereby going into a sleeping state. FIG. 28 shows signal transmission and reception procedures in this case.
In case when the sleep mode is applied in the IBSS in the IEEE 802.11, a certain time frame from the TBTT is defined as an ATIM (Announcement Traffic Indication Message) window. In the time frame of the ATIM window, all MTs belonging to the IBSS are operating the signal reception procedures. Any MT operating in the sleep mode is basically capable of receiving in this time frame.
In case when each MT has information to be sent to a certain designated MT, it is possible to transmit ATIM packets to the designated MT after transmitting the beacon in the ATIM window, thereby informing the receiver side that the MT holds information to the designated MT. The MT that received the ATIM packets keeps the receiver operating until the end of reception from the ATIM packets transmitting station.
In the example as shown in FIG. 28, there are three MTs, i.e., a STA1, a STA2 and a STA3 in the IBSS. When the TBTT arrives in FIG. 28, each MT of the STA1, STA2 and STA3 sets its backoff timer while monitoring the media condition over the random interval. The example in FIG. 28 shows the case when the timer of the STA1 disappears earlier than the others and transmits the beacon. Since the STA1 transmits the beacon, the other SAT2 and STA3 which receive the beacon are refrained from transmitting the beacon.
Also, in the example in FIG. 28, the STA1 holds the transmission information addressed to the STA2 while the STA2 holds the transmission information addressed to the STA3. At this time, both the STA1 and STA2 set the backoff timer again while monitoring the respective media condition over the random time interval after transmission/reception of the beacon. In the shown example, since the timer of the STA2 disappears earlier, the ATIM message is transmitted first from the STA2 to the STA3. Upon receiving the ATIM message, the STA3 feeds the receiving ACK (acknowledge) packet back to the STA2. Upon completion of transmission of the ACK from the STA3, the STA1 further sets the backoff timer while monitoring the respective media condition over the random time interval. When the timer disappears, the STA1 transmits the ATIM packet to the STA2. The STA2 feeds the ACK packet of receiving the ATIM packet back to the STA1.
Upon performing communication of these ATIM packet and ACK packet in the ATIM window, the STA3 operates the receiver in order to receive information from the STA2 in the subsequent interval while the STA2 operates the receiver in order to receive information from the STA1.
In the above procedures, communication stations which do not receive the ATIM packet in the ATIM window or do not hold information to be transmitted to any other station can shut off the power of the transmitter/receiver until the next subsequent TBTT, thereby reducing power consumption.
Incidentally, information processing equipment such as personal computers (PCs) come into wide use and offices are in a working environment where a variety of equipments are included, thereby including communication stations spreading all over and constructing a plurality of networks in overlapped with one another. Under such circumstances, there arises a need for an access control so that communications between terminals do not conflict with one another.
In a packet communication or the like in which communication requests are in random and burst manner, it is typical to adopt a common channel system in which a plurality of terminal stations share the same frequency channel. Since communication requests from terminal stations are random in the common channel system, it is most likely that signals from a plurality of terminal stations collide with one another, thereby degrading communication quality. In order to avoid such signal collision, a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) system is widely adopted because it comprises a relatively simple mechanism. In the ad-hock communication system as described hereinabove, a direct, asynchronous information transmission system is applied in accordance with the access procedures based on the CSMA/CA in order to detect that the communication from the own station does not collide with the others.
On the other hand, in case of transmitting real-time and continuous data such as dynamic images and the like which require to transmit data periodically at a constant interval, bandwidth must be guaranteed. In such a case, bandwidth is guaranteed by giving a period of time for each communication station constituting the network to prioritize transmission/reception.
However, if a constant prioritized transmission/reception right is given in case of relatively small number of communication stations, there is a problem to cause useless latency. It is to be noted that the term “latency” means herein a waiting time or a delay time from the time when an instruction is sent to the time when an actual result or response is received. Reduction of such latency can be realized by shortening the waiting time before starting communication or increasing the transmission rate.